Laminates comprising one or more layers of polyimide and one or more layers of substrate material may be used for a variety of applications, for example as reinforcing materials. In addition, laminates of the type in question, in the form of polyimide-coated metal foils, are used for printed electrical circuits. In that case, use is made of the flexibility and outstanding mechanical, thermal and electrical properties of the polyimides. This is because the laminates are frequently exposed to high temperatures during further processing, for example during soldering or drilling. The laminates also have to satisfy stringent requirements in regard to their electrical and mechanical properties.
Laminates comprising only one substrate layer of metal or a metal alloy and a layer of polyimide, so-called single clads, may be used for printed electrical circuits. The same applies to multilayer laminates, so-called double clads or multilayer circuits, which comprise several metal layers and/or several polyimide layers. In certain cases, however, multilayer laminates are superior to single clads. Thus, in the case of printed circuits for example, it is often necessary to make printed conductor lines which intersect one another. The high packing densities often required cannot be obtained where single clads are used, but only where double clads or multiclads are used. The present invention is concerned with multilayer laminates which are eminently suitable for the production of double clads and multiclads. In this context, double clads are understood to be laminates comprising two (metallic) substrate layers, while multiclads are understood to be laminates comprising more than two (metallic) substrate layers.
Laminates containing polyimides and substrate materials are known. In this case, the polyimide layers are often bonded to the substrate by a conventional adhesive. Thus, U.S. Pat. No. 3,900,662 for example describes the bonding of polyimide to metal by an acrylate-based adhesive. Use is also made of this possibility in the laminates described in U.S. Pat. No. 3,822,175. However, it has been found that where conventional adhesive, for example based on acrylate, epoxide, polyamide, phenolic resin, etc. are used, the laminates in which the polyimide is bonded to the metal by an intermediate layer of one of these adhesives do not show entirely satisfactory properties which meet the strigent demands often imposed. This is attributable on the one hand to the fact that the thermal stability of the adhesive is inferior to that of the polyimide. Thus, adhesive can be a serious obstacle to the further processing of the laminates because it becomes fluid at the high temperatures applied during further processing and can thus adversely affect the quality of the product. In addition, the adhesives are inferior to the polyimides in their electrical properties, i.e. in their insulating effect.
On account of the disadvantages of laminates comprising layers of conventional adhesives between polyimide and metal, multilayer laminates have been proposed in which the polyimide is bonded directly to metal, i.e. without a layer of adhesive. Thus, German Offenlegungsschrift No. 32 15 944 for example describes laminates in which two metal layers are bonded by an intermediate layer of polyimide. The polyimide used in this case is obtained predominantly from diphenyl tetracarboxylic acid and may be bonded to a metal foil by applying high temperature and pressure. In other words, the polyimide is formable. It has been found, however, that formable polyimides or polyimides which are soluble in phenolic solvents are inferior in their thermal stability to fully aromatic, intractable, i.e. no longer formable, polyimides insoluble in phenolic solvents.
Because of the disadvantages of adhesives, single clads of a substrate to which a fully aromatic, intractable polyimide insoluble in phenolic solvents is directly bonded have already been proposed. These single clads show excellent mechanical, thermal and electrical properties.
Starting out from these single clads, it would be desirable to produce double clads or multilayer laminates (multiclads) which likewise consist only of substrates and fully aromatic, intractable polyimides and which would thus show the same mechanical, thermal and electrical properties. However, it has been found that two or more single clads of this type cannot be directly bonded to one another or one single clad directly bonded to a metallic substrate, i.e. without an intermediate layer of adhesive, because it is not possible to apply another layer of substrate or another single clad to the fully cured polyimide layer without a coupling layer in such a way that high peel strength, i.e. high adhesion between the polyimide and the additional layer, is obtained. Although, on the other hand, application of the other layer of substrate before the polyimide has been completely cured is possible in principle and leads to an increase in peel strength, bubbles can be formed in the polyimide layer because volatile constituents, for example water, have to escape during its curing and the release of these volatile constituents can be impeded by the additional layer of substrate.